This work is concerned with the analysis of gne organization and expression in oogenesis and early development in Xenopus laevis. Recombinant DNA molecules (cDNA clones) have been synthesized using as a starting template poly(A) containing RNA from oocytes or stage 41 tadpoles. These clones have been divided into a number of categories which include sequences which are expressed in oogenesis and disappear through early development and sequences which are absent during oogenesis and appear during early development. These clones are being analyzed in detail to define more precisely the way in which gene expression is modulated during early development. In addition, genomic clones have been isolated which are complementary to these cDNA clones. This will permit the analysis of the sequence organization of genes expressed during oogenesis and early development. Finally, gene expression during oogenesis will be assayed directly by comparing the sequence concentration of individual mRNAs as a function of oogenetic stage by making use of individual recombinant molecules.